1. Field of the Invention
The present invention relates to a control apparatus and a control method for an internal combustion engine, for calculating an exhaust gas recirculation (EGR) flow rate in an internal combustion engine on which variable valve timing (VVT) mechanisms are mounted.
2. Description of the Related Art
In general, in order to preferably control an internal combustion engine (hereinafter also referred to as “engine”), it is important to precisely calculate an air amount taken into a cylinder of the engine (hereinafter referred to as “cylinder intake air amount”), to thereby carry out fuel control and ignition timing control in response to the cylinder intake air amount.
On this occasion, as a method of calculating the cylinder intake air amount, there are generally known a method of using an airflow sensor (AFS) for calculation (hereinafter referred to as “AFS method”) and a method of using an intake manifold pressure sensor for calculation (hereinafter referred to as “speed density (S/D) method”).
Specifically, in the AFS method, the AFS mounted on an intake pipe of the engine on a throttle upstream side is used to measure an air flow rate passing through an mounted portion of the AFS (hereinafter referred to as “AFS intake air amount”), and the cylinder intake air amount is calculated while considering a response lag on a throttle downstream side.
Moreover, in the S/D method, an intake manifold pressure sensor mounted to a surge tank and an intake manifold on the throttle downstream side of the intake pipe is used to measure a pressure in the intake manifold (hereinafter referred to as “intake manifold pressure”), and the cylinder intake air amount is calculated based on the intake manifold pressure and the engine rotational speed.
Note that, there are known a technology of switching between the AFS method and the S/D method depending on an operation state by using both the AFS and the intake manifold pressure sensor, and a technology of increasing a precision of the response lag on the throttle downstream side by using the intake manifold pressure even in the AFS method.
Moreover, regarding the fuel control, generally excellent controllability can be obtained by carrying out acceleration/deceleration correction and feedback control so as to inject a fuel amount for attaining a target air/fuel ratio mainly for the cylinder intake air mount.
On the other hand, the ignition timing control needs to be carried out so as to attain an ignition advance angle providing the maximum engine output (minimum spark advance for best torque (MBT)) depending on not only the engine rotational speed and the cylinder intake air amount but also other factors such as an engine temperature, a knock generation state, fuel characteristics, and an EGR ratio, which is a ratio between an EGR amount and the AFS intake air amount.
Moreover, out of the above-mentioned factors affecting the MBT, for example, the engine temperature can be detected by an engine coolant temperature sensor, the knock generation state can be detected by a knock sensor, and the fuel characteristics of whether the fuel is a regular gasoline or a high-octane gasoline can be determined depending on the knock generation state.
On this occasion, regarding the EGR rate, there are known external EGR control for controlling an EGR amount in an EGR passage connecting between the exhaust pipe and the intake pipe and internal EGR control for controlling an EGR amount by using exhaust gas remaining in the cylinder. The external EGR control and the internal EGR are commonly used simultaneously.
Specifically, in the external EGR control, an EGR valve is mounted on the EGR passage, and the EGR amount is controlled based on an opening degree of the EGR valve. Moreover, in the internal EGR control, VVT mechanisms for changing valve opening/closing timings of an intake valve and an exhaust valve are mounted, and an overlap period during which the intake valve and the exhaust valve are simultaneously opened is changed based on the valve opening/closing timings, to thereby control the EGR amount caused by the exhaust gas remaining in the cylinder.
In the following, a simple notation of EGR refers to the EGR introduced by the external EGR control, and a flow rate (hereinafter referred to as “EGR flow rate”) Qegr of the EGR gas introduced by the external EGR control can be calculated by, for example, Expression (1) based on a calculation equation for the flow rate passing through a nozzle in the compressible fluid dynamics.Qegr=Segr·αegr·σegr·ρegr  (1)
In Expression (1), Segr denotes an effective opening area of the EGR valve, αegr denotes the sound speed of the EGR gas, σegr denotes a dimensionless flow rate of the EGR gas, and ρegr denotes the density of the EGR gas. Moreover, the effective opening area Segr of the EGR valve is calculated as a correlation value of the EGR valve opening degree.
Moreover, the sound speed αegr of the EGR gas can be calculated by using Expression (2).αegr=√{square root over (κegr·Regr·Tegr)}  (2)
In Expression (2), κegr denotes the specific heat ratio of the EGR gas (such as 1.38), Regr denotes the gas constant of the EGR gas (such as 0.282 [kJ/(kg·K)]), and Tegr denotes the temperature of the EGR gas. Note that, as the temperature Tegr of the EGR gas, a temperature Tex in the exhaust pipe (hereinafter referred to as “exhaust temperature Tex”) may be used.
Moreover, the dimensionless flow rate σegr of the EGR gas can be calculated by using Expression (3).
                              σ          egr                =                                            2                                                κ                  egr                                -                1                                      [                                                            (                                                            P                      b                                                              P                      ex                                                        )                                                  2                                      κ                    egr                                                              -                                                (                                                            P                      b                                                              P                      ex                                                        )                                                                                            κ                      egr                                        +                    1                                                        κ                    egr                                                                        ]                                              (        3        )                                (                              ∵                                          P                b                            ≤                              P                ex                                              ,                                                    (                                                      P                    b                                                        P                    ex                                                  )                            choke                        =                                          (                                  2                                                            κ                      egr                                        +                    1                                                  )                                                              κ                  egr                                                                      κ                    egr                                    -                  1                                                                    )                                        
In Expression (3), Pb denotes the intake manifold pressure and Pex denotes the pressure in the exhaust pipe (hereinafter referred to as “exhaust pressure”). It is conceivable that the exhaust pressure Pex is approximated by the atmospheric pressure Pa for a naturally-aspirated engine.
Moreover, when Pb/Pex is less than (Pb/Pex)choke, the state is in a choke area, and the dimensionless flow rate σegr of the EGR gas on this occasion has the same value as σegr@choke for (Pb/Pex)choke.
Moreover, the density ρegr of the EGR gas can be calculated by using Expression (4).
                              ρ          egr                =                              P            ex                                              R              egr                        ·                          T              egr                                                          (        4        )            
In Expression (4), as described above, as the temperature Tegr of the EGR gas, the exhaust temperature Tex may be used.
On this occasion, as a method of calculating the EGR flow rate by using Expressions (1) to (4), for example, there are proposed methods disclosed in Japanese Patent Nos. 4019265 and 3861046.
Specifically, in Japanese Patent No. 4019265, an opening area of an EGR passage is calculated based on an opening degree of an EGR valve, and an EGR flow rate is calculated from a map storing a relationship of an EGR flow rate under a predetermined atmospheric pressure corresponding to an intake pipe internal pressure correlation value, which is acquired by correcting an intake pipe internal pressure detected value with an atmospheric pressure detected value, and an engine rotational speed based on the intake pipe internal pressure correlation value and a detected value of the engine rotational speed. Then, a density correction coefficient is calculated based on the atmospheric pressure detected value, and the EGR flow rate is calculated based on the opening area of the EGR passage, the EGR flow rate, and the density correction coefficient. On this occasion, the above-mentioned relationship corresponds to the product of Expressions (2) to (4) under the predetermined atmospheric pressure.
Moreover, in Japanese Patent No. 3861046, a tentative EGR gas flow rate is calculated by using Expressions (1) to (4), and such a correction value that increases toward 1 as a pressure difference between an upstream gas pressure corresponding to the exhaust pressure and a downstream gas pressure corresponding to the intake manifold pressure increases is acquired. Then, an error contained in the tentative EGR gas flow rate, which is caused by a pipe friction between an exhaust gas recirculation pipe corresponding to the EGR passage and the EGR gas, is corrected by multiplying the tentative EGR gas flow rate by the correction value, to thereby calculate the flow rate of the EGR gas flowing from the exhaust gas recirculation pipe to an intake passage.
However, the related art has the following problems.
Expressions (1) to (4) hold true for a non-viscous ideal gas assuming isentropic conditions. It is considered that, in such a state that the air, which is a mixture of nitrogen and oxygen, and the EGR gas, which is a mixture of nitrogen, carbon dioxide, and vapor, move in the internal combustion engine, influence of a change in entropy and the specific heat is not so great.
Therefore, it is considered that when Expressions (1) to (4) are applied with use of the effective opening area reflecting influence of the viscosity, which corresponds to the product of a flow rate coefficient and the opening area, the EGR flow rate can be calculated without such a large error.
Nonetheless, Japanese Patent No. 4019265 has a description that the product of Expressions (2) to (4) is calculated for each engine rotational speed, and Japanese Patent No. 3861046 has a description that the error caused by the pipe friction between the EGR passage and the EGR gas is corrected by such a correction value that increases toward 1 as the pressure difference between the exhaust pressure and the intake manifold pressure increases.
Those descriptions are considered to suggest that, due to a problem specific to the EGR passage connecting between the exhaust pipe and the intake pipe of the engine, the EGR flow rate cannot be calculated with sufficient precision only by simply applying Expressions (1) to (4).
Thus, the inventor (s) of the present invention calculated the EGR flow rate by applying Expressions (1) to (4) to an engine including an intake VVT mechanism and an exhaust VVT mechanism and configured to carry out external EGR control, and confirmed that the calculation error in the EGR flow rate is increased not only by the engine rotational speed and the pressure difference between the exhaust pressure and the intake manifold pressure, but also by VVT phase angles.
Further, it was found out that, out of the intake VVT and the exhaust VVT, particularly the phase angle of the exhaust VVT has a great influence on the calculation result of the EGR flow rate. On the engine including the intake VVT mechanism and the exhaust VVT mechanism and configured to carry out the external EGR control, there is such a problem that the EGR flow rate cannot be precisely calculated only by the correction based on the engine rotational speed and the pressure difference between the exhaust pressure and the intake manifold pressure.